This invention relates to a process for preparing a radiation image storage panel employing a stimulable phosphor.
Radiation images like X-ray images are often used in diagnosis of diseases. The methods have traditionally employed light-sensitive silver halide materials and a fluorescent screen to visualize images. However, recently, methods directly taking out images from phosphors are being used.
The recent methods include, for example, a method in which the radiation transmitted through a subject is absorbed by a phosphor, and thereafter this phosphor is excited by light or heat energy to cause the radiation energy accumulated by being absorbed to radiate as fluorescence. The fluorescence is then detected and formed into an image.
Specifically, U.S. Pat. No. 3,859,527 and Japanese Unexamined Patent Publication No. 12144/1980 disclose radiation image storage methods in which a stimulable phosphor is used and visible light or infrared rays are used as stimulating light.
This method employs a radiation image storage panel (hereinafter often referred to as "panel") comprising a support, and formed thereon, a stimulable phosphor layer (hereinafter often referred to simply as "phosphor layer"). Radiation transmitted through a subject falls on the phosphor layer to accumulate radiation energy corresponding to the radiation transmission degree of each areas of the subject, thereby to form an image. Thereafter the phosphor layer is scanned with the stimulating light to cause the radiation energy accumulated in the areas to radiate and convert into light, thus obtaining an image formed of signals based on the strength of the light.
The image finally obtained may be reproduced as a hard copy, or may be reproduced on a CRT.
The panel having the phosphor layer used in this radiation image storage method is required to have low graininess of the image and yet have high sharpness, in addition to high radiation absorption and light conversion (including both, herein called "radiation sensitivity"). The radiation sensitivity is experimentally measured by using emission intensity.
However, in general, the panels having the phosphor layer are prepared by coating on a support a dispersion containing a particulate stimulable phosphor having a particle diameter of about 1 to 30 .mu.m and an organic binder, followed by drying. The result is in a low charge density for the stimulable phosphor (charge weight: 50%), requiring that the phosphor layer be applied thickly to achieve sufficiently high radiation sensitivity dielectric constant.
However, the sharpness of the image in the above radiation image storage method has a tendency to be greater with a decrease in the layer thickness of the phosphor layer of a panel, so that the phosphor layer must be made thinner to improve the sharpness.
Also, the graininess of the image in the above radiation image storage method depends on the spatial fluctuation of radiation quantum number (i.e. quantum mottles) or the structural disorder of the phosphor layer of a panel (i.e. structure mottles), so that making the layer smaller thickness of the phosphor layer brings about decrease in the radiation quantum number to be absorbed in the phosphor layer. This, in turn, causes an increase in the quantum mottles, or brings the structural disorder to be realized to cause a lowering of image quality. Thus, the phosphor layer must be made with a large thickness to reduce the graininess of images.
In other words, in the conventional panels, the sensitivity to radiation and the graininess of images show quite opposite tendencies to the sharpness of images with respect to the layer thickness of the phosphor layer. Accordingly, the above panels have been prepared with a compromise between the sensitivity to radiation, the grainines and the sharpness to a certain extent.
Incidentally, as well known the sharpness of images in the conventional radiography depends on the extent of the instantaneous emission (emission at the time of irradiation of radiations) of the phosphor present in a fluorescent screen. In contrast therewith, however, the sharpness of images in the radiation image storage method utilizing the above-mentioned stimulable phosphor does not depend on the extent of stimulated emission of the stimulable phosphor present in the panel, but depends on the stimulating light in said panel.
Therefore, if the stimulated emission by the stimulating excitation light irradiated at a certain time (ti) is comprised only of the emission from picture elements (xi, yi) on said panel on which the stimulating light had been actually irradiated at the time (ti), the emission, whatever extent it has, does not affect the sharpness of the resulting image.
From such a viewpoint, there have been devised some methods for improving the sharpness of radiation images. They are exemplified by a method in which a white powder is mixed into the phosphor layer of a panel as described in Japanese Unexamined Patent Publication No. 146447/1980, and a method in which a panel is so colored that the average reflectance at the stimulating excitation wavelength region of a stimulable phosphor is smaller than the average reflectance at the stimulated emission wavelength region of said stimulable phosphor. These methods, however, necessarily result in an extreme lowering of the sensitivity if the sharpness is improved, and can not be said to be preferable methods.
In view of the disadvantages and the conflict between properties as stated above, Japanese Unexamined Patent Publication No. 73100/1986 proposes a panel comprising a phosphor layer, and a preparation process thereof, free of any binder, formed by a vapor phase build-up method such as vacuum deposition. According thereto, the phosphor layer of the above panel contains no binder, so that the charge weight in the phosphor layer can be remarkably improved and also the directivity of the stimulating excitation light and stimulated emission of the phosphor layer can be improved, resulting in improvement in the sensitivity to radiation of the panel and the graininess of images, and at the same time improvement in the sharpness of images.
The panel containing no binder can be prepared by the vapor phase build-up methods such as sputtering, CVD and vapor deposition, but, when taking account of the production cost, the vapor deposition can be said to be the most preferable method.
However, when vacuum deposition methods or the like are used, a problem arises due to the differing vapor pressure of components when heated in the crucible, for deposition. The result is that the composition of the deposited stimulable phosphor is different from the composition charged into the crucible. The usual result is that the panel has lower sensitivity to radiation.
In other words, although the method for the vapor phase build-up of the stimulable phosphor may bring about a number of advantages as stated above, neglecting the conditions for vaporizing the phosphor when the stimulable phosphor is formed or a heat treatment is effected may result in unexpected difficulties.
For example, researches made by the present inventors in regard to Tl-activated stimulable RbBr:Tl phosphors revealed that the instantaneous emission intensity is constant when Tl content is in the range of from 10.sup.-2 to 10.sup.O mol %, and it is therefore unnecessary to be greatly careful of its compositional ratio as a generally available phosphor so long as the Tl content is kept within a constant range (FIG. 8).
However, the stimulated emission important to the panel has a peak near 3.times.10.sup.-2 mol % with its intensity greatly lowering in the vicinity thereof.
Accordingly, the difference in the vapor pressure between an activator and a stimulable phosphor matrix in carrying out vapor phase build-up of the stimulable phosphor by vapor deposition or the like may result in the activator being deposited on the panel support precedently or retardatorily to the phosphor matrix. If thereby the concentration of the activator comes to differ in the thickness direction in the phosphor layer or the activator is not added, resulting in deviation from its optimum concentration, it may follow that the phosphor layer is retarded rather than enhanced with respect to the desired activity expected by the use of the activator or that it exhibits no sufficient emission characteristics.
In preparing the above panel utilizing the stimulated emission, also particularly important for obtaining a panel having high sensitivity is to precisely control the concentration of the activator in a post-treatment (heat treatment).
In this connection, RbBr exhibits a vapor pressure of 1 mmHg at 777.degree. C., and TlBr, 10 mmHg at 522.degree. C., thus having a great difference in the vapor pressure
There has been little reported in this regard, and examples frequently seen are activated stimulable phosphors having poor performances in spite of the activator densities that are optimum as a whole.